1. Field of the Invention
The present invention relates to a DC/DC converter configured to convert a DC voltage, and more particularly, to a DC/DC converter including a timer circuit.
2. Description of the Related Art
A related-art DC/DC converter is now described. FIG. 11 is a circuit diagram illustrating the related-art DC/DC converter.
The related-art DC/DC converter includes a comparator 504, an RS-FF circuit 113, a drive circuit 110, a reference voltage generation circuit 503, a timer circuit 501, an NMOS transistor 108 and an NMOS transistor 109, which are output transistors, a capacitor 107, a coil 106, resistors 103, 104, and 502, a ground terminal 100, an output terminal 102, and a power supply terminal 101.
The comparator 504 has an inverting input terminal for inputting a divided voltage VFB obtained by dividing an output voltage Vout of the output terminal 102 and a non-inverting input terminal for inputting a reference voltage obtained by adding a ripple voltage, which depends on a power supply voltage and the output voltage Vout, and a slope voltage, which changes with a predetermined slope, to each other. The comparator 504 outputs a signal corresponding to the result of comparison. When the divided voltage VFB is higher than the reference voltage, the comparator 504 outputs a signal of Low to a set terminal of the RS-FF circuit 113. When the divided voltage VFB is lower than the reference voltage, the comparator 504 outputs a signal of High to the set terminal of the RS-FF circuit 113. A reset terminal of the RS-FF circuit 113 is supplied with a signal output from the timer circuit 501. An output signal is output from a Q terminal of the RS-FF circuit 113 in accordance with the output signal of the comparator 504 and the output signal of the timer circuit 501. The drive circuit 110 controls on and off of the NMOS transistors 108 and 109 in response to the signal from the RS-FF circuit 113, to thereby generate the output voltage Vout from the output terminal 102 (see, for example, Japanese Patent Application Laid-open No. 2011-182533).
In the related-art DC/DC converter, however, there is a problem in that, when the output voltage increases in a light load state, if control of turning off the output transistor in order to decrease the output voltage is continued, the timer circuit cannot secure a predetermined ON-time required for resuming to a normal operation because of the decreased output voltage, with the result that it takes time for the DC/DC converter to resume to the normal operation. Further, there is a problem in that a large ripple may be generated in the light load state as compared to normal continuous mode control.